Every once in a while, the art market gives permission for a "do-over," a kind of contemporary art-historical mulligan where a younger artist gets to make new versions of widely known artworks by some senior figure. So it was with the pioneering Conceptual artist Lawrence Weiner, whose words-as-art technique was revived first by 1980s artist Jenny Holzer and now by the British artist Martin Creed. Weiner was the provocative philosopher, and Holzer the feminist moralist, but what is Creed?